The invention relates to a method for operating a plant having at least one open- and/or closed-loop heating control device, an open- and/or closed-loop heating control device, and a plant having an open- and/or closed-loop heating control device of this kind.
Industrially manufactured products are often thermally treated using heaters. Even slight variations in the heating process may have an extremely adverse effect on product quality. In order to increase the quality of a heat-treated product, it is important to be able to focus the required energy very precisely in time and space. This is achieved using special open- and/or closed-loop heating controllers which ensure highly precise control of heating elements. Frequently used as heating elements are resistive loads in the form of radiant heaters, in particular infrared heaters.
Blow molding plants, for example, usually have radiant heater panels for heating preforms. The radiant heaters (infrared heaters) are supplied electrically via a switching element connected into the power supply, controlled in an open-/closed-loop manner and monitored in respect of their power output by an open- and/or closed-loop heating control device.
For this purpose the open- and/or closed-loop heating control device frequently receives setpoint values for the heat output of the connected heating elements from a higher-order open- and/or closed-loop control device, e.g. a programmable logic controller (PLC), via an open field bus. The setpoint values can be provided, for example, in the form of absolute setpoint values, of setpoint values relating to a maximum power or of setpoint values relating to a rated power. The power can relate, for example, to a heat output to be delivered or an electric power to be drawn by heating elements. Control signals for the switching elements are then derived from these setpoint values in the open- and/or closed-loop heating control device using a predefined open- and/or closed-loop control algorithm. However, the setpoint values may also be already available in the form of pulse packets or percentages of half cycles per unit time (e.g. per second) from which control signals for the switching elements can then be derived directly. The control signals are then used to control in an open- or closed-loop manner the switching state of the switching elements and therefore the heat outputs of the heating elements. For simplification and better understanding, all these setpoint values will be referred to as “setpoint values for a heat output” in the following description.
The triggering of the switching elements and therefore the open- or closed-loop control of the switching state or more specifically of the heat output can take place, for example, using phase control or half cycle control using zero power switching elements. Switching elements such as semiconductors (e.g. solid state relays), for example, can be used for this purpose.
Known open- and/or closed-loop heating control devices of this kind usually have an electric output power of approx. 0.5 to 5 kW (for a supply voltage of 230 Vdc) per heating element and a max. total electric output power of 500 kW.
In a plant, a plurality of such open- and/or closed-loop heating control devices are usually supplied (mainly together with other plant equipment) from a common plant power supply system. Because of their electric power requirement, the open- and/or closed-loop heating control devices sometimes place a heavy load on the power system.
In order to optimize the power supply system in respect of its load, it is necessary to know an instantaneous power draw of the open- and/or closed-loop heating control devices or more specifically of the heating elements controlled by them. For this purpose it is already known to explicitly determine the electric power in the open- and/or closed-loop heating control devices on the basis of measured variables (current through the heating elements, voltage across the heating elements), either separately for each heating element (or rather each heating channel) or altogether for each open- and/or closed-loop heating control device. The disadvantage of this is that a complex and therefore costly measuring device is necessary in order to acquire the measured variables.